The present invention relates to alternator structures for munitions in general and artillery projectiles in particular which are fired from a rifled bore and have a spin signature in order to provide electrical energy for guidance of the projectile or for arming the fuse. Heretofore alternator structures were used with non-spin projectiles such as mortars, bombs, and rockets in which velocity in flight was used as a separate spin signature. Such alternators are designed with an air duct assembly or vanes which respond to an air stream for imparting a rotary motion to the rotor assembly of the alternator device. There exists, however, a need for confining an alternator assembly within the confines of a rifled bore of a gun of from 35 mm to 40 mm, completely contained in the round to be fired and which can sustain severe acceleration and high gravity forces in the course of being fired from such guns and at the same time provide electrical power immediately upon the emergence of the round from the gun barrel in order to power an internal guidance system for precision guided munitions.